This invention relates to an optical device-related adhesive for bonding optical elements in optical isolators and other optical devices used in optical communication systems and optical instruments, and optical devices using the same.
When light from a light source is transmitted by way of an optical system, the light is reflected by the end face of the optical system back to the light source. In the case of signal transmission through an optical fiber, for example, when light emitted by a laser light source is projected to the end face of the fiber through lenses, the majority enters the fiber as light being transmitted, but a portion thereof is reflected by the lenses and the fiber end face back to the laser light source to disturb the oscillation of the laser, causing noise generation. To eliminate such noise, optical isolators are used in the prior art.
Referring to FIG. 1, one exemplary optical isolator is shown as comprising a Faraday element 1 to which a polarizer 2 and an analyzer 3 are joined with adhesive layers 4, 4. The assemblage is inserted into a cylindrical magnet 5.
The recent demand to the optical isolator is to improve the durability of its optical elements under hot humid conditions. There is a need for a technique capable of forming a bond between optical elements which endures under hot humid conditions.
If the bond established is poor, the adhesive layer between optical elements will deteriorate in a thermal shock test between xe2x88x9245xc2x0 C. and 85xc2x0 C. or during long-term storage at 85xc2x0 C./RH 85% of the assembly. Then emergent light from the optical isolator has an optical axis offset. When this optical isolator is incorporated in a semiconductor module, the optical fiber has an increased coupling loss. Still worse, the adhesive layer can fail.
On the other hand, in order to establish a reliable bond between optical elements, it is necessary to metallize the entire surface of the optical elements except the light transmitting portions for solder joint and form a metal joint, or to form a low-melting glass joint. However, due to complex steps included, these techniques suffer from low manufacturing yields and increased costs.
An object of the invention is to provide an adhesive for optical devices capable of forming a bond which remains intact under hot humid conditions or thermal cycling conditions and is impact resistant. Another object of the invention is to provide a highly reliable optical device using the adhesive.
It has been found that an adhesive in the form of a liquid epoxy resin composition comprising (A) a liquid epoxy resin and (B) 20 to 90% by weight based on the entire composition of an inorganic filler having an average particle size from more than 1 xcexcm to 20 xcexcm and containing up to 1% by weight of a fraction of particles having a particle size of at least 45 xcexcm is improved in adhesion performance to crystalline materials such as polarizers and analyzers so that optical isolators and optical devices having improved reliability are obtainable. Most preferably, spherical silica is used as the inorganic filler.
If an adhesive containing a substantial amount of large particles is applied to optical elements of very small size, the elements cannot be maintained on a horizontal level and the adhesive cannot penetrate into narrow gaps. This adversely affects the reliability of the optical device. Using an inorganic filler having a controlled particle size as represented by an average particle size from more than 1 xcexcm to 20 xcexcm and inclusion of 1% by weight or less of a particle fraction having a particle size of at least 45 xcexcm, there is obtained an adhesive which can penetrate into narrow gaps.
When a specific amount of the inorganic filler having the controlled particle size is present in the liquid epoxy resin composition, the composition is reduced in expansion/contraction rate by virtue of the inorganic filler, preventing interfacial separation between the composition and optical elements due to thermal expansion/contraction in a thermal shock test between xe2x88x9245xc2x0 C. and 85xc2x0 C. As a result, the liquid epoxy resin composition according to the invention is effective for improving thermal shock resistance and adhesion to optical elements. When a silicone-modified resin is further formulated in the composition, the composition is reduced in modulus of elasticity by virtue of the silicone, so that the above-described advantages are more effectively achieved.
Accordingly, the invention provides an optical device-related adhesive for bonding optical elements to substrates, in the form of a liquid epoxy resin composition comprising (A) a liquid epoxy resin and (B) an inorganic filler having an average particle size from more than 1 xcexcm to 20 xcexcm and containing up to 1% by weight of a fraction of particles having a particle size of at least 45 xcexcm. The inorganic filler is present in an amount of 20 to 90% by weight of the entire composition. Preferably the liquid epoxy resin composition further includes (C) a silicone-modified resin.
Also contemplated herein is an optical device comprising an optical element which is bonded to a substrate using the adhesive.